The present invention relates to an electrophotographic photoreceptor, in particular, one that is suitable for use with printers, copiers, etc. and which shows high sensitivity to light from LED and semiconductor lasers.
Electrophotographic photoreceptors having high sensitivity to visible light are used extensively with copiers, printers, etc. Most common photoreceptors that are used in these applications are inorganic photoreceptors provided with light-sensitive layers that are chiefly composed of inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide. However, such inorganic photoreceptors are not completely satisfactory in such characteristics as photosensitivity, heat stability, moisture resistance and durability that are required of electrophotographic photoreceptors to be used with copiers, printers, etc. For instance, selenium will crystallize upon heating or exposure to dirt such as sebum, often leading to deterioration of the photoreceptors that use it as a photoconductor. Photoreceptors that use cadmium sulfide are low in moisture resistance and durability, whereas those using zinc oxide are poor in durability. Further, photoreceptors using selenium or cadmium sulfide are subject to great restraints during manufacture and handling.
With a view to solving these problems with inorganic photoconductive materials, attempts have been made to use a variety of organic photoconductive materials in the light-sensitive layers of photoreceptors and active R&D efforts are being made today along this line. For example, JP-B-50-10496 (the term "JP-B" as used hereinafter means an "examined Japanese patent publication") describes an organic photoreceptor having a light-sensitive layer containing poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone. However, even this photoreceptor is not completely satisfactory in terms of sensitivity and durability. To overcome this problem, an electrophotographic photoreceptor of a functionally separated type has been developed. In this type of photoreceptor, the light-sensitive layer is divided into a carrier generation layer and a carrier transport layer, with a carrier generation material and a carrier transport material being cntained in the respective layers. Since the carrier generating and transporting capabilities are individually fulfilled by different materials, suitable materials can be selected from a broad range of choices and hence a photoreceptor having desired characteristics can be obtained fairly easily. For these reasons, it is anticipated that an organic photoreceptor having high sensitivity and durability can be produced using the concept of function separation.
A great number of materials have so far been proposed that can be effectively used as carrier generating materials in the carrier generation layer of a functionally separated electrophotographic photoreceptor. An illustrative inorganic material that can be used is amorphous selenium (see JP-B-43-16198). A carrier generation layer containing this amorphous selenium is used in combination with a carrier transport layer containing an organic carrier transport material. However, the carrier generation layer comprising amorphous selenium suffers the problem of deterioration of its characteristics because as already mentioned, selenium will crystalize upon heating or exposure to sebum. Organic materials can also be used as carrier generation materials and they are exemplified by organic dyes and pigments. For example, the use of bisazo compounds in light-sensitive layers has been proposed in JP-A-47-37543 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-55-22834, JP-54-79632, JP-A-56-116040, etc. These and other known bisazo compounds show fairly good sensitivity in either the short or medium wavelength range but their sensitivity in the long wavelength range is too low to justify their use with laser printers which employ semiconductor lasers as light sources that are anticipated to insure high reliability.
Gallium-aluminum-arsenic (Ga.Al.As) light-emitting devices which are extensively used in semiconductor lasers operate at wavelengths long than 750 nm. Many studies have so far been conducted in order to obtain electrophotographic photoreceptors that have high sensitivity to light in the wavelength range longer than 750 nm. For instance, it has been proposed that spectral sensitizers that extend the wavelength of sensitivity to the longer range should be added to light-sensitive materials such as Se and CdS having high sensitivity to the visible range. However, as already mentioned, Se and CdS do not have satisfactory resistance to environmental factors such as temperature and moisture and hence are not considered to be ideal materials.
Although many organic photoconductive materials are known, their sensitivity is generally limited, as mentioned above, to the visible region shorter than 700 nm and very few organic materials have satisfactory sensitivity to the range longer than 700 nm. The rare exception is phthalocyanine compounds which are known to have sensitivity to a longer wavelength range than other organic photoconductive materials. An example of such photoconductive phthalocyanine compounds is .alpha.-titanyl phthalocyanine which is described in JP-A-61-239248. This .alpha.-titanyl phthalocyanine has peaks of the Bragg angle at 7.5.degree., 12.3.degree. , 16.3.degree. , 25.3.degree. and 28.7.degree. with respect to X-rays of Cu K.alpha. at 1.541 .ANG.. However, it has low sensitivity and its potential stability during cyclic use is so poor that it will readily experience fogging in electrophotographic processes which rely upon reversal development.
The titanyl phthalocyanine of type II described in JP-A-62-67094 has a peak of the Bragg angle (2.theta.) at 27.3.degree. and the peak of maximum intensity is located at about 7.5.degree. in the 2.theta. range of 6.degree.-8.degree.. However, this titanyl phthalocyanine of type II has low chargeability and suffers from such disadvantages as high levels of dark decay and residual potential.
Thus, the prior art titanyl phthalocyanine compounds of the .alpha.-form and type II have not been completely satisfactory for use in electrophotographic photoreceptors from various aspects such as chargeability, sensitivity and potential stability during cyclic use.